Condensate drain subcooler for moisture separator and reheater

ABSTRACT

A moisture separator and reheater for separating moisture from wet steam and heating it using condensing-type tube bundles operating under saturated conditions incorporates a special heat exchanger in the condensate drain line to subcool the condensate and prevent instability or cavitation in the drain. The subcooling heat exchanger boils liquid taken from the moisture separator and reheater cycle to perform the condensate cooling.

United States Patent 1191 Reed 1451 July 10, 1973 CONDENSATE DRAINSUBCOOLER FOR MOISTURE SEPARATOR AND REHEATER [75] Inventor: William G.Reed, Cumberland Foreside, Maine [73] Assignee: .General ElectricCompany,

Schenectady, N.Y.

22 Filed: Oct. 12,1971

211 App1.No.: 188,090

[52] US. Cl 122/483, 60/73, 165/113 51 1111.0. F22g 1/04 58 Field ofSearch 122/483; 60/73;'

[561' 1 References Cited UNITED STATES PATENTS 7/1970 Viscovich et a1.6(1/73 4/1970 Durrer 122/483 4/1971 Rabas 122/483 Primary ExaminerAlbert W. Davis, Jr. Att0meyWi1liam C. Crutch'er et a1.

57 ABSTRACT A moisture separator and reheater for separating moisturefrom wet steam and heating it using condensingtype tube bundlesoperating under saturated conditions incorporates a special heatexchanger in the condensate drain line to subcool the condensate andprevent instability or cavitation in the drain. The subcooling heatexchanger boils liquid taken from 'the moisture separator and reh'eatercycle to perform the condensate cooling. 5

2 Claims, 1 Drawing Figure DRY REHEATED STEAM OUTLET I 1 SATURATED STEAMINLET VENT SAT. STEAM INLET CONDENSATE OUTLET WET STEAM INLET CONDENSATEDRAIN SUBCOOLER FOR MOISTURE SEPARATOR AND REHEATER BACKGROUND OF THEINVENTION This invention relates generally to moisture separator andreheater apparatus used in steam turbine power plants, and moreparticularly relates to improvements for the condensate drain lines fromcondensing-type tube bundles.

Moisture separator and reheater devices, hereafter referred to as MSRs,are well-known in turbinegenerator power plants and serve to separatemoisture from wet steam leaving a turbine section and then to reheat thesteam to a superheated condition before it enters the next turbinesection. The moisture separating and the reheating functions of MSRshave either been performed in separate pressure vessels or more recentlythe two functions have been combined into a single pressure vessel orshell.

In order to perform the heating function, cycle steam is often takenfrom the powerplant cycle at a higher energy level, usually near asaturated condition, and is then condensed in tube bundles inside theMSR..Since the condensatedrains from a header in a substantiallysaturated state, any transient pressure or temperature differences inthe drain line can cause flashing into steam with resulting instabilityproblems or blocking of the drain lines.

Accordingly, one object of the present invention is to provide animproved construction for an MSR condensate drain system. I 7

Another object of the invention is to provide an improved constructionfor drain lines from condensingtype heat exchangers. I

SUMMARY OF THE INVENTION Briefly stated, the invention comprisesimprovements in an MSR of the type using condensing-type reheatersections, by means of incorporating a subcooling heat exchanger in thecondensate drain line from the reheater. The heat exchanger drawsseparated liquid from the MSR cycle by means of the natural pressuredifference through the MSR. The liquid boils in the heat exchanger,thereby subcooling the condensate an reducing its tendency to flash intosteam.

DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to thedrawing, the MSR includes a horizontal cylindrical shell 1 with endpieces 2, 3 welded in place to form a pressure vessel. Wet steam from aturbine section enters at an inlet such as 4 from the bottom and dryreheated steam exists at an outlet such as 5 on the top. A portion ofthe length of shell 1 is omitted in order to enlarge the scale of thedrawing,

but in practive there are a number of such inlets and outlets 4, 5 andthe MSR shell 1 may be on the order of 50 to 80 feet long and 12 feet indiameter.

Although the details of the particular MSR construction are immaterialto the present invention, there being many different types known in theart, a brief descriptionfollows of the type of MSR to which the, pres-'ent invention is applied, more complete details being set forth inacopending application, Ser. No. 190,538 filed Oct. 19, 1971 filed inthe names of T. Carnavos, W. Hubble and K. Woods and assigned to thepresent assignee. 7

Dual banks of corrugated moistureseparatorelements, one bank being shownat 6, serve to remove moisture from steam entering inlet 4. The moistureflows to the bottom of the shell and is removed through suitableconduits 5a. A first stage reheating tube bundle 7 and a second stagebundle 8 are disposed on top of one another. The steam flows over tubebundles 7, 8 to be reheated before it exits at the top of the vesselthrough outlet 5.

As more fully described in the aforementioned pending application, firstand second stage reheaters 7, 8 are of the type having transverselyextending internal headers 9, 10, respectively, with a verticalpartition, such as '1 l, separating the inlet and outlet sides. Theindividual tubes of the bundles extend the length of the pressurevessel, make a horizontal bend at the end as indicated at 12, and thenreturn to the header of the opposite side of partition 11. Both of theheaders'9, 10 are similar in construction and completely containedwithin the MSR. Header 9 is shown in cross section to indicate theinternal features, whileheader 10 is shown in full'to illustrate thatthere is a manway 13 on either end thereof to permit access for tubemaintenance and inspection.

Entry into the pressure vessel for the reheater piping is through theend wall 2, and the internal piping generally includes pipe sections ofsubstantial length running transversely across the pressure vessel, asindicated at 14, in order to allow for thermal transients withoutoverstressing the pipes and headers. The piping for the first stagereheater 7 comprises a steam inlet line 15, a condensate drain outletline 16, and a vent line 17. Similarly, piping for the second stagereheater includes steam inlet line 18, condensate outlet line 19, andvent line 20. The steam inlet lines 15, 18 are connected to one end ofthe respective headers 9, 10 while the condensate outlet lines 16, 19and vent lines 17, 20 are connected to vtheopposite sides of theirrespective headers on the other side of the vertical dividing partitions11.

Steam for the reheaters at a substantially saturated condition is bledfrom the turbine power plant cycle and introduced to first stage inlet15. Similarly, steam at a higher temperature and pressure, but alsosubstantially saturated, is introduced to inlet line 18. By way ofexample, but no limitation, steam entering 18 may be saturated'and onthe order of 1,000 psi (545F) while steam entering inlet 15 may be" onthe order of v 500 psi. (467 F).'The wet cycle steam being treated byconventional type, well known to those skilled in the art, and can be ofextended surface applied to the drain lines within an enclosure, or canbe shell-in-tube with either the condensate drain fluid or the coolingfluid running through the tubes. The construction shown, for purposes ofillustration, in the cross section of heat exchanger 21 comprises atransverse shell 23 running beneath header 9 and connected thereto by apipe section 24. A transversely extending tube bundle 25 is supportedwithin shell 23 and has headers on opposite ends for connecting to awater inlet line 26 and a steam out- OPERATION OF THE INVENTION Inoperation, wet cycle steam has liquid separated from it by separatorbanks 6, and the steam is reheated by passing over first and secondstage reheaters 7, 8. Steam bled at substantially saturated conditionfrom higher pressure and temperature energy levels in the cycle flowsinto the headers 9, 10, passes through the reheater tubes and returns toheaders 9, 10,- condensing along the way. Substantially saturatedconditions exist inthe outlet headers.

The normal pressure drop encountered by the main cycle steam flowingthrough the MSR over the separators and reheater tubes may be on theorder of psi. Thus, the pressure difference between that of the water inwell 30 and that of heat exchanger steam outlet 31 is sufficient to pumpwater through the heat exchangers. Since this water is substantially insaturated condition at these lower pressures, it boils inside tubebundles 25 which are surrounded by the higher temperature liquidcondensate draining from headers 9, 10. This serves to subcool thecondensate below its saturation'temperature. The steam from the tubebundles 25 exits to rejoin the cycle steam at outlet pipe 31.

Although the invention has been illustrated with respect to an MSR withinternal headers, it is equally applicable to units with externalheaders where headers such as 9, are disposed on the outside of thepressure shell. In this case, heat exchangers 21, 22 are also external,as is the connecting piping. I

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications may be made therein, and it is intended. to coverin the appended claims all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. In a shell containing a separator and reheater of the type havingmeans separating a first liquid portion from a first lower pressure,lower temperature vapor and also having means for reheating said firstvapor, said reheating means comprising a condensing heat exchangeradapted to condense a second higher pressure, higher temperature vaporflowing therethrough into a second condensate portion, the improvementcomprismg:

boiling heat exchanger means connected to pass said first separatedliquid portion in heat exchange relationship with said second condensateportion within said shell so as to cause said first liquid portion toboil, thereby subcooling said second condensate portion.

2. In a moisture separator and reheater of the type having meansseparating a first water portion from a first lower pressure, lowertemperature wet steam flow, and having at least one condensing tubebundle adapted to reheat said first steam flow by condensing a secondhigher pressure, higher temperature saturated steam flow passingthroughsaid tubes into a second condensate portion, said first steamflow undergoing a pressure drop through the moisture separator, theimprovement comprising:

a condensate drain conduit for removing said second condensate portionfrom the reheater tubes,

a boiling heat exchanger disposed in heat exchange relationship withsaid condensate drain conduit and having an inlet and an outlet,

first conduit means connected between said heat exchanger inlet and saidfirst separated water portion, and

second conduit means connected between said heat exchanger outlet and alower pressure region downstream of said reheater, whereby the pressuredifference across said moisture separator reheater causes the firstseparated water portion to flow through the boiling heat exchanger tosubcool the second condensate portion.

* I8 I! i

1. In a shell containing a separator and reheater of the type havingmeans separating a first liquid portion from a first lower pressure,lower temperature vapor and also having means for reheating said firstvapor, said reheating means comprising a condensing heat exchangeradapted to condense a second higher pressure, higher temperature vaporflowing therethrough into a second condensate portion, the improvementcomprising: boiling heat exchanger means connected to pass said firstseparated liquid portion in heat exchange relationship with said secondcondensate portion within said shell so as to cause said first liquidportion to boil, thereby subcooling said second condensate portion. 2.In a moisture separator and reheater of the type having means separatinga first water portion from a first lower pressure, lower temperature wetsteam flow, and having at least one condensing tube bundle adapted toreheat said first steam flow by condensing a second higher pressure,higher temperature saturated steam flow passing through said tubes intoa second condensate portion, said first steam flow undergoing a pressuredrop through the moisture separator, the improvement comprising: acondensate drain conduit for removing said second condensate portionfrom the reheater tubes, a boiling heat exchanger disposed in heatexchange relationship with said condensate drain conduit and having aninlet and an outlet, first conduit means connected between said heatexchAnger inlet and said first separated water portion, and secondconduit means connected between said heat exchanger outlet and a lowerpressure region downstream of said reheater, whereby the pressuredifference across said moisture separator reheater causes the firstseparated water portion to flow through the boiling heat exchanger tosubcool the second condensate portion.